Various solutions to the above problem have been proposed in the background art mostly comprising the use of so-called trap-lock joints or trap-lock metal composite interfaces (MCIs). When using a trap-lock MCI the common method of manufacture is filament-winding. Numerous textbooks on composite materials and technology describe filament-winding in detail. Here it is sufficient to state that filament-winding is a manufacturing process wherein resin impregnated fibre bundles (tows) are placed along geodetics in helical patterns on top of a mandrel or liner. After the filament-winding operation is finished the resin is cross-linked (cured). For a so-called riser joint, the metallic liner to which two metallic end pieces are butt welded, constitutes the said mandrel/liner. Depending on the actual stresses occurring during operation the tubular section itself should contain appropriate amounts of fibres in angle plies in both a close to axial direction and hoop wise. A trap-lock joint as known from the background art is basically one or more circumferential grooves into which one or more angle ply layers of reinforcing material are bent and attached as is shown schematically in FIG. 1 of the present application. After the fibre bundles have been placed into the grooves, they may be attached and fixed into the grooves by hoop winding further fibres tangentially on top of said angle ply layers in the groove, thereby locking the said angle ply layers into place. The cross-sectional area of the load carrying angle ply layers is one of the main parameters determining the strength of the lock or interface. The above method may be further refined as proposed in EP097049 wherein is described the use of a plurality of traplock grooves. EP097049 proposes a method in which the traplocks are of varying depth in order for the load to be equally distributed in each groove, and to avoid one of the groves bearing the entire load of the fibres. GB2415727 describes an end connector at a metal to composite interface with accompanying seal for the termination of a composite riser. The traplock joint resembles to a large degree the solution as described in EP097049. Further solutions have been provided in WO2004044372 and U.S. Pat. No. 6,050,612 in which more and more advanced solutions and improvements are proposed.
US20030067167 describes a joint for the connection of fibre-reinforced composite pipes comprising a male part and a female part provided on two end parts of two pipes to be connected, characterized in that said male part and said female part have an axial modulus of elasticity which substantially increases proportionally from their free ends to their connection with said pipes.
U.S. Pat. No. 6,676,169 describes a method including the attaching of a connector having at least one trap to a segment of composite pipe comprising a plurality of filament fibers, winding the plurality of filament fibers across the end connector, wherein tension is continuously maintained on the filament fibers so that the filament fibers bridge across the at least one trap.
U.S. Pat. No. 5,702,109 provides an expandable high-pressure flexible-tube device, in particular for sealing piping in gas or oil fields, comprising two end pieces and a tubular element composed of elastomeric material comprising at least two sheets of cords wound in a helix about the longitudinal axis of said element, as a small angle of about 10 DEG to 15 DEG relative to said axis, characterized in that each sheet is stranded and comprises, in the warp direction, very strong textile cords.
U.S. Pat. No. 5,327,963 describes a coupling device adapted for attachment to an elastomeric tubular body having looped reinforcement fibers, as part of a tubular assembly. The device includes a coupling member, a stem, and a mechanism for retaining the elastomeric tubular body in a sealing engagement with the stem, and a mechanism for retaining the elastomeric tubular body in sealing engagement with the stem when connected therewith. The retaining mechanism maintains this engagement by being interposed with loops of the reinforcement fibers to limit axial movement of the reinforcement fibers and the elastomeric tubular body relative to said stem.
U.S. Pat. No. 4,755,406 describes a method for forming a composite material tube connection and a connection device comprising application thereof.
There are several problems regarding the abovementioned methods, the main problem being that these systems are unsuitable if there are large compressive or axial forces acting upon the fibers. If a large compressive force acts upon a traplock joint, the fibres may loosen from their attachments to the load carrying face of the groove, and the fibres may delaminate. The bending of the fibres into the grooves may also be problematic as the bend zone when subject to large compressive or axial forces may be a crack initiator for the fibres. The section of the stiff end piece in which the grooves are arranged may be suitably constructed so as for the bend angle of the fibres into the grooves to limit the amount of fibre bending, however this entails the widening of this section of the pipe, and solely reduces the bending stresses and does not eliminate the strains. Although fibres or fibre bundles may have very high tolerances for axial loads, they are thus often less well adapted to bending.